In recent years energy saving is an important development issue in construction machines such as hydraulic excavators and wheel loaders. To achieve the energy saving of the construction machine, energy saving of the hydraulic system itself is essential. In this regard, examination is being made on employment of a hydraulic closed circuit in which a hydraulic pump having two delivery ports and being capable of bidirectional delivery (hereinafter referred to as a “bidirectional delivery hydraulic pump”) is connected to a hydraulic actuator in closed circuit connection to directly drive the hydraulic actuator. In such a hydraulic closed circuit, there is no pressure loss caused by control valves. There is no flow loss either since only a necessary flow is delivered from the hydraulic pump. Further, it is possible to recover potential energy of the actuator and energy at times of deceleration (energy regeneration). As above, the energy saving of the hydraulic system is made possible by employing a hydraulic closed circuit for the hydraulic system.
Hydraulic cylinders of the single rod type (single rod hydraulic cylinders) are generally used as the hydraulic cylinders in construction machines. In order to connect such a single rod hydraulic cylinder to a hydraulic pump in closed circuit connection, it is necessary to absorb a flow rate difference that is caused by a pressure-receiving area difference between the head-side chamber and the rod-side chamber of the hydraulic cylinder. In the conventional technology, a charge pump and a low-pressure selection valve (flushing valve) are generally used to absorb the flow rate difference (see FIG. 2 of Patent Literature 1, for example). There have also been disclosed hydraulic systems absorbing the flow rate difference without using a charge pump or a low-pressure selection valve in FIGS. 1 and 3 of the Patent Literature 1 and in Patent Literatures 2 and 3.
In the hydraulic system disclosed in FIGS. 1 and 3 of the Patent Literature 1, two bidirectional delivery hydraulic pumps are arranged with their drive shafts connected to each other. The two delivery ports of one hydraulic pump are connected to the head-side chamber and the rod-side chamber of a hydraulic cylinder, respectively. One delivery port of the other hydraulic pump is connected to the head-side chamber, and the other delivery port is connected to a hydraulic fluid tank.
In the hydraulic system disclosed in the Patent Literature 2, a hydraulic closed circuit including a hydraulic cylinder and a hydraulic pump connected together in closed circuit connection is connected to an open circuit. At times of extension of the hydraulic cylinder, the head-side chamber of the hydraulic cylinder is supplemented with hydraulic fluid supplied from a hydraulic pump on the open circuit's side. At times of retraction of the hydraulic cylinder, surplus hydraulic fluid is returned from a hydraulic line on the low pressure side of the hydraulic cylinder to the hydraulic fluid tank via a low-pressure selection valve in the same way as in the conventional technology.
In the hydraulic system disclosed in the Patent Literature 3 (FIGS. 2 and 7), a hydraulic closed circuit including a boom cylinder and a hydraulic pump connected together in closed circuit connection is connected to an open circuit. At times of boom raising (at times of extension of the hydraulic cylinder), the hydraulic fluid is supplied from a hydraulic pump on the open circuit's side to the head-side chamber of the boom cylinder (high pressure side), while a hydraulic line on the rod side (low pressure side) of the hydraulic closed circuit is connected to the hydraulic fluid tank via a switching valve and a relief valve. At times of boom lowering (at times of retraction of the hydraulic cylinder), surplus hydraulic fluid is returned to the hydraulic fluid tank via the switching valve and the relief valve.